1. Field of the Invention
This invention relates to Radio Frequency (RF) and microwave signals and in particular the generation of high power microwave signals for use in high resolution radar and Directed Energy applications. Because microwave signals fall within the range of RF frequencies, all subsequent references to “RF or microwave signals” and “RF and/or microwave signals” are intended to mean a reference to Radio Frequency signals.
2. Discussion of Prior Art
Shaw H. J., Elliott, B. J., Harker K. J., and Karp A., Microwave generation in pulsed ferrites, J. App. Phys., Vol 37, No. 3, 1966 details a method for converting energy from a pulsed magnetic field to a microwave signal in a waveguide. Shaw's ‘Pulsed Magnetic Microwave Generator’ relies on impulse-driven gyromagnetic precession in a small Yttrium Iron Garnet (YIG) sphere to convert energy from a ‘pump pulse’ to a microwave signal. A small spherical (approximately 1 mm diameter) specimen of magnetic material is placed in a microwave waveguide cavity. A strong, constant magnetic bias field is applied to the magnetic sphere in order to align magnetic moments within the magnetic material. Subsequently a strong pulsed magnetic field is applied to the sphere at some angle to the constant bias field. The pulsed magnetic field causes realignment of the magnetisation vector in the sphere on a timescale that is similar to the rise time of the pulsed field. If the rise time is sufficiently short the magnetisation vector does not simply follow the pulsed field but precesses around the direction of the applied field. This gyromagnetic precession persists for a time that is related to damping processes in the magnetic material. The precessing magnetisation vector in the magnetic material constitutes an oscillator at the gyromagnetic precession frequency. The magnetic sphere is located in a waveguide cavity so that energy from the gyromagnetic oscillator is coupled into the waveguide as an electromagnetic signal. Consequently the magnetic sphere is used as an impulse-excited transducer that converts energy from the pulsed magnetic field to a microwave signal in the waveguide.
Disadvantages of this method of generation of microwave signals include; the amount of energy and power that can be converted to a microwave signal is limited by the requirement for a small spherical specimen of magnetic material, usually YIG, to achieve fully temporally and spatially coherent precession and, because the pulsed magnetic field is produced by passing a large pulsed current through an external coil, the ratio of microwave energy produced to the energy stored in the coil is inherently low, as the pulsed magnetic field is applied to a large volume compared to the volume of the sphere.
An object of the present invention is to provide an alternative method and apparatus for generating pulsed RF or microwave signals.